A few weeks ago, I mentioned that I would try to help in spotting some JO forgeries. This is a difficult and complex subject and I am definitely an amateur. The Showa overprints were carried out in the Chinese press at Kuching. Contrary to what was known before, there are definitely minor variations within a sheet of overprints. Looking at the pair of 1Y Buddha stamps above, one could see the minor differences in thickness and length of strokes in the japanese characters.
The 5s stamp is genuine but I believe the 10s has a fake overprint. Again there are some minor differences in the overprint.
Looking at the back, one could see an impression has been made at the back by the overprint on the 5s stamp but is fairly flat on the other stamp even though one could see the ink coming through. It does not show up that clearly on the scan.
Again a genuine 8s stamp with the same 10s stamp with forged overprint.
At the back of the 8s stamp, one could see some impression made by the overprint. I believe this is the easiest way to spot a fake Showa overprint even though it is definitely not fool proof. Over the weekend, I was attending the Philatex fair in London. One dealer was offering a good selection of the 20s Mount Fiji Showa stamp at a fair price. It is my favourite stamp in the set. I was going to buy but on examining the back and finding no indentation, I changed my mind. By the way, if you are buying used, make sure the date is August or September 1945. Because they use at least 2 different calender systems, I have on idea what it is supposed to look like. But they are still philatelic rather than of normal usage.
Update Repeating this exercise with my small collection of 3 line overprints has yielded some disturbing results. The "Borneo Kita" inscription is exactly the same as the bottom line of the 3 line overprint. My 2 tarnished looking copies has definite indentation but my fresh looking stamps including a $1 used copy do not look so promising. The chop was also applied at the Chinese Press in Kuching.